Seismic Refraction And Gravity Investigation Of A Tunnel Route Near Columbus, Ohio

A sewer tunnel is under construction in the northwest section of the Columbus, Ohio<br>metropolitan area. The tunnel is 10 kilometers long and 20 meters below the ground surface.<br>Bedrock is limestone with till overburden of variable thickness. During tunneling through limestone<br>an unexpected zone of soft material was encountered, resulting in a shutdown of the tunneling<br>machine. A long, expensive delay followed while a vertical shaft was excavated and the soft material<br>was tunneled by hand. Concern that other problem zones might be encountered led to contracting<br>a geophysical survey to identify additional problem areas in advance of tunneling.<br>The tunnel route is through an affluent residential area with extensive infrastructure<br>development. After evaluating the background noise that would affect various geophysical methods<br>and desiring to minimize the disruption to traffic flow and local residents, we decided that<br>microgravity and seismic refraction were the most appropriate methods. Station spacing was about<br>6 meters for both gravity stations and geophone locations. The gravity survey was reduced to the<br>Bouguer anomaly with modeling to interpret terrain effects in areas where significant valleys crossed<br>the profile or where the edge of a parallel valley came close to the line. The regional gravity trend<br>was removed graphically.<br>A strong, non-explosive, minimally-intrusive seismic source was required so we chose an<br>elastic wave generator. This provided enough energy to see first arrivals over above background<br>noise to offsets of 75 meters. We used 24 single-geophone traces for the seismic refraction. The<br>source points were typically one station off the ends of each spread. The line was advanced in 12<br>station steps to provide a continuous bedrock profile with some redundancy. We picked first breaks<br>‘and analyzed the data as a two-layer case with ray tracing software.<br>Gravity and seismic bedrock-elevation profiles generally show good correlation with the<br>results of the extensive geotechnical borings that had been done. By selecting the appropriate density<br>contrast between the overburden and bedrock, we constructed a bedrock elevation profile which<br>agreed well with boring information at most locations. The seismic refraction and boring logs were<br>used to determine the bedrock profile. The gravity profile was examined to identify zones where the<br>gravity was lower than expected on the basis of bedrock topography alone. These zones were<br>considered to be areas where the density of the material below the bedrock surface was abnormally<br>low, and were targeted as potential bad rock zones. Additional subsurface investigation was<br>recommended at these locations to allow advanced planning for tunnel problems in the area.